I agree with @unman that rules different from those that can be expressed with qvm-firewall would not work off the shelf.
@ryzenpro as the unikernel is only a firewall, you won’t be able to run nano, nor save rule files there. But it is possible to modify the kernel code to add a special redirection. For example, a user recently requested a nat redirect (from sys-net to a dedicated domU client), and I suggested maintaining a fork (see GitHub - palainp/qubes-mirage-firewall at manual-nat-bind).
As I don’t understand your rules, if custom-forward is similar to “forward packets received on this port to this client”, as the previous description, you should be able to modify the code. If not, the solution may be more complex and I can encourage you to open an issue on the github repository.